Currently, metastatic castration-resistant prostate cancer (CRPC) is invariably and unfortunately a lethal disease. The vast majority of CRPC is dependent upon and driven by androgen receptor (AR) signaling. While recent advances have resulted in the development of second generation therapeutics targeting the androgen signaling axis (e.g. enzalutamide and abiraterone), the responses to these new agents are often not durable. Thus, there is a clear and urgent need to develop completely new therapeutic approaches directed against the AR axis. Recently, our team identified a novel strategy for targeting the AR signaling axis via inhibition of bromodomain containing 4 (BRD4), a conserved member of the bromodomain and extraterminal (BET) family of transcriptional co-activators (In Revision at Nature). We discovered that AR physically interacts with BRD4, and that treatment with the BET-bromodomain inhibitor JQl disrupts the AR-BRD4 interaction as well as AR recruitment to target gene loci. Compared to the direct AR antagonist enzalutamide, BET bromodomain inhibitors are much more effective in inhibition of AR-mediated gene transcription, including induction of AR targets such as TMPRSS2:ERG and PSA. Significantly, JQ1 was also more effective than enzalutamide in inhibiting the growth of CRPC xenografts. Based upon our findings, we hypothesize that BET bromodomain inhibition is a promising approach for targeting the AR axis and for treating advanced prostate cancer. Our goal in this SPORE project is to develop a potent BET-bromodomain small-molecule inhibitor (BET inhibitor) with optimized in vivo properties for the treatment of advanced prostate cancer through the following specific aims: Aim 1: Develop a highly potent and selective BET inhibitor with optimized in vivo properties Aim 2: Interrogate the AR-BRD4 signaling axis with novel BET inhibitors. Aim 3: Establish the efficacy of BET bromodomain inhibitors using in vivo and ex vivo tumor models, and develop biomarkers of response. Upon successful completion of the Aims, we expect to nominate promising candidate(s) that could be developed further for clinical use to treat metastatic CRPG. The ultimate goal of this project is introduce BET bromodomain inhibitors as a novel therapy for CRPC based on a unique understanding of its mechanism of action.